Any simple template can be used really, that can later be (optionally) automatically processed by specialized client.

Something as simple as putting PULLREQUEST, MERGED, REJECTED etc as the first word of messages in which

the repo is tagged in the usual way.

Simple enough that it doesn't even require specialized software until a repo grows too big.

A couple of dozen issues can be managed manually. Couple of hundred? Use a tool that filters messages...

For example, an open pull request is one that doens't have a reply anywhere down the reply tree, coming from the repo's pubkey starting with the word MERGED or REJECTED,

An open issue is one that doesn't have the reply CLOSED anywhere down the tree coming from the repo account.

Etc... Hand writable, easy to filter w/ software.

Here is an example:

nostr:note1xxt8m3gywkve06lyfgx02mrlusq0qzn4vsrzjp25wnr803n03zfsdql3u6